High voltage shutdown protection circuit with bias arrangement to decrease the voltage shutdown point with increasing load

ABSTRACT

A current and voltage regulated DC to DC converter is designed with protection features to permit its operation in parallel with like DC to DC converters. The features are designed to assure shutdown protection against overcurrent and overvoltage conditions and to assure that each converter will contribute a current to the common load. The protection features include a selective high voltage shutdown to shut down only the converter causing an overvoltage and an overcurrent protection circuit to supersede the normal current regulation in response to a fault condition. A reverse current shutdown circuit protects the common load from faults and short circuits internal to the converter circuit. These protection features permit the converters to operate in parallel without shutting down the entire system should an individual converter malfunction.

United States Patent [191 1 3, 84,893 Rando 5] Jan. 8, 1974 [5 HIGHVOLTAGE SHUTDOWN 3,743,887 7/1973 Keough 317 33 sc x PROTECTION CIRCUITWITH BIAS ARRANGEMENT TO DECREASE THE VOLTAGE SHUTDOWN POINT WITHINCREASING LOAD Primary ExaminerWilliam M. Shoop, Jr. Attorney-W. L.Keefauver et al.

[75] Inventor: Robert Rando, Brooklyn, N.Y. ABSTRACT 73 Assignee; lTelephone Laboratories, A current and voltage regulated DC to DCconverter I t d, M Hi1] N is designed with protection features to permitits operation in parallel with like DC to DC converters. The [22] Flled:1973 features are designed to assure shutdown protection [21] App]. No.:322,406 against overcurrent and overvoltage conditions and to assurethat each converter will contribute a current to the common load. Theprotection features include a [52] US. Cl. 321/11, 321/19 Selective highvoltage Shutdown to shut down only the [51] Int. Cl. H02m 1/18 converterCausing an overvoltage and an overcurrem [58] Field of Search 321/11, 19protection circuit to supersede the normal current ulation in responseto a fault condition. A reverse cur- [56] References and rent shutdowncircuit protects the common load from NITE STATES PATENTS faults andshort circuits internal to the converter cir- 3,376,487 4/1968 Bixby321/19 X Cult. These protection features permit the converters 3,461,3748/1969 321/19 X to operate in parallel without shutting down the entire8 ,85 /1 321/19 X system should an individual converter malfunction.3,602,804 8/1971 323/20 3,614.587 10/1971 Schwarz 321/19 X 6 Claims, 3Drawing Figures W. fij E1 129 /I33 I 126 154 122 I 132 |07 SHUTDOWNCURRENT CURREN CCT SIGNAL SIGNAL GL JRRETI T AMP AMP MONITOR I50 IDISCONNECT [O47 CIRCUIT CURRENT REGULATOR CCT PATENTEU JAN 8l974 sm 3 0F3 I HIGH VOLTAGE SHUTDOWN PROTECTION CIRCUIT WITH BIAS ARRANGEMENT TODECREASE THE VOLTAGE SHUTDOWN POINT WITH INCREASING LOAD BACKGROUND OFTHE INVENTION This invention relates to converter circuits, and, moreparticularly, to converter circuits connected in parallel to a commonbus. It is specifically concerned with overvoltage protection and theshutdown of a malfunctioning converter.

In situations where a plurality of converters are connected in parallelto a common bus, the converters will all share a common output voltage.Should an overvoltage occur on the bus due to the failure of voltageregulation of one of the converters, protective circuits individual tothe other converters may respond to the overvoltage. Hence, convertersothan than a particular malfunctioning converter may shut down inresponse to their overvoltage protective circuit since overvoltage iscommon to all converters.

Another problem associated with voltage regulated converters connectedin parallel is that while all will deliver a regulated voltage they willnot share equally in supplying the load current to the bus. Some ofthese converters may supply their full regulated output current. Otherconverters while generating their regulated output voltage may supply nocurrent at all to the common bus.

It is therefore an object of the invention to protect convertersconnected in parallel to a common bus from an overvoltage condition.

It is also an object to shut down only malfunctioning convertersresponsible for a high voltage condition on a common bus.

It is another object of the invention to establish a current outputrange of operation for each converter so that all the converters willsupply a minimum current to the bus.

SUMMARY OF THE INVENTION In accordance with the invention where aplurality of converters are connected in parallel to' a common bus, eachconverter includes a voltage monitoring circuit to continuously monitor,its votlage output. Should any converter malfunction and produce anovervoltage, that converter is shut down selectively. Each converterincludes a selective shutdown circuit which shuts down only theconverter producing an overvoltage condition. This selective shutdowncircuit generates a bias signal proportional to the output load currentof the converter. This bias signal is used to modify the response of thevoltage monitoring circuit. The overvoltage threshold of the voltagemonitoring circuit is controlled to linearly decrease in proportion toan increasing output current of the converter. Hence at a full loadcurrent the overvoltage threshold at which shutdown occurs will be lowerthan at a small load current.

It is apparent that if the voltage regulation of a converter fails thatconverter will supply a higher proportion of loadcurrent to the bus. Thecurrent demand of the remaining converters to supply load current to thebus is reduced and hence the threshold of their overvoltage protectioncircuit will be increased. Only the individual converter responsible forthe overvoltage condition is shut down since it has the lowestovervoltage threshold.

A feature of the invention is a voltage regulation low load uptiltfeature which requires each parallel connected converter to contributeload current to the bus. Each converter includes circuitry to increaseits voltage regulation threshold at low load conditions. The value atwhich the voltage is regulated on each converter is increased inresponse to a decrease in the output load current below a specifiedcurrent threshold. This is acomplished by deriving a bias signalresponsive to the output load current of the converter and utilizing itto modify the regulated voltage value at low load current. Hence thiscircuitry requires each converter to contribute at least a minimum loadcurrent to the bus.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the invention maybe readily ascertained by reference to the following detaileddescription hereinbelow and the drawings described wherein:

DETAILED DESCRIPTION The DC to DC converter disclosed in block diagramform in FIG. 1 is designed to be operated in parallel with otherconverters. This converter includes both overvoltage and overcurrentprotection features designed to facilitate parallel operation. Theovercurrent protection circuitry includes means to supersede the normalcurrent regulation of the converter in order to provide increased outputcurrent capacity to facilitate clearing of faults on the output bus.Shutdown of the converter in case of a sustained overload on the bus isalso provided for protection of the converter. The high voltageprotection shutdown is selective so that only the converter casuing anovervoltage is shut down. In addition. the converter includes means toassure the current contribution of each converter to the bus and toprovide reverse current protection. These characteristics may be readilyascertained by inspection of the current-voltage diagram of theconverter characteristics in FIG. 2 which are described in detailhereinbelow in conjunction with the description of the power convertercircuit disclosed in FIG. 1.

POWER CIRCUIT The power circuit portion of the converter comprises twoinput terminals 121 and 122 to which a directcurrent voltage source maybe connected. The output of the direct-current source is smoothed toreduce variations by an input filter comprising an inductor 123 and acapacitor 117. The power circuit itself comprises the two controlledswitching devices and 116 which in the illustrative embodiment aresilicon controlled rectifiers. The two silicon controlled recitifiers115 and 116 are poled in the same direction and connected to oppositeterminals of the primary winding 118 of the power transformer 110. Theprimary winding 118 is center tapped to form two winding segments 109and l 19. The winding segments 109 and 119 form two LC resonant networksin series with the parallel combination of the capacitors 125 and 124.

' Each silicon controlled rectifier, which are subsequently referred toherein as SCR diodes, is connected in series with an LC resonantnetwork. The SCR diode 115 is connected in series with-the windingsegment 109, which is one-half of the primary winding 1 18, and thecapacitors 125 and 124. The SCR diode 1 16 is connected in series withthe winding segment 119 and the capacitors 124 and 125. The SCR diodes115 and 116 conduct alternately to charge and discharge the capacitors125 and 124. The SCR diodes 115 and 116 are driven by simultaneous gatetrigger signals which are applied to the trigger leads of the SCR diodes115 and 116 by a blocking oscillator 135. These signals are applied tothe trigger leads of the sCR diodes l and 116 via leads 186 and 187,respectively. At the beginning of each conduction period one SCR diodeis reversebiased and the other SCR diode is forward-biased.

If, for example, SCR diode 115 is forward-biased, it will conduct inresponse to the trigger signal supplied by the blocking oscillator 134,via lead 186. The SCR diode 115 will conduct current through the windingsegment 109 to charge the capacitors 125 and 124. During this period theSCR dio'de 116 is reverse-biased by the voltage across the capacitor125. As the trigger inputs of the SCR diodes 115 and 116 are pulsed, thepower circuit will resonate at a frequency determined by the parametersof the capacitors 124 and 125 and the winding segments 109 and 119. Thecapacitor 125 eventually charges to the point where the SCR diode 115becomes reverse-biased and it ceases to conduct. In the meantime thecapacitor 124 has charged to a voltage at which the SCR diode 116 isforward-biased and it will conduct in response to the next trigger signainput.

CONVERTER OUTPUT CIRCUIT When either of the SCR diodes 115 and 116 isconducting, the initial polarity across the output winding 108 of thepower transformer 110 is such that output diode 126 is back-biased.

As the capacitors 124 and 125 charge or discharge, depending upon therespective conducting states of the SCR diodes 11S and 116, the voltageacross the secondary winding 108 of the power transformer 110 reversesand attains the regulated output voltage level. At this point the outputdiode 126 becomes forwardbiased, the conducting one of the SCRdiodescommutates, and the energy stored in the transformer 110 is dischargedinto the output filter of the converter as a ramp of current. The outputfilter comprising the capacitor 127 and the inductor 128 smooths theoutput signal applied to the output terminals 131 and 132. The outputvoltage of the converter is directly proportional to the switchingfrequency of the SCR diodes 115 and 116. The converter output isregulated by controlling the frequency of the blocking oscillator.

The output signals of the converter circuit are monitored to supplyfeedback signals to control the current and voltage outputs of theconverter by controlling the Frequency of operation of the blockingoscillator 135. The output current is monitored by sensing the voltageirop across a current sensing resistor 129 connected in series with theoutput terminal 131. A series connected 'esistor 133 and capacitor 130shunt the output termi- 1813 131 and 132. The capacitor 130 filters theoutput signal and the resistor 133 is utilized to detectreversecurrents. The voltage across the sensing resistor 129 is monitored bythe two current signal amplifiers 101 and 103. A high voltage monitorcircuit 114 and a voltage regulating amplifier 111 have monitoringterminals connected to the output terminals 131 and 132.

VOLTAGE REGULATlON The voltage output of the converter is regulated inresponse to the voltage regulating amplifier 111 which monitors theoutput voltage at output terminals 131 and 132 of the converter, vialeads 141 and 142. The voltage regulating amplifier 111 sums theconverter output voltage with a reference voltage to produce anamplified direct-current error signal. This signal is applied through adiode gate 185 to a comparator circuit 113 via lead 186. The comparatorcircuit compares this signal with a signal supplied by the syncamplifier 112.

The input to the sync amplifier 112 is supplied via lead 144 from oneterminal 197 of the secondary winding 108 of the power transformer 110.The output of the sync amplifier 112 is the amplified version of theconverters output ripple signal. This signal is compared by thecomparator circuit 113 with the amplified direct-current error signalsupplied by the voltage regulating amplifier 111. The comparator circuit113 supplies a signal, via the gate 187, to control the frequencyof theblocking oscillator 1 35 and hence regulate the output voltage. Thenormal voltage regulation characteristic of the converter is shown bywaveform a in FIG. 2.

POWER LIMIT CONTROL A second signal is supplied to the blockingoscillator 135 by the power limit circuit 188. The power limit circuit188 is connected to the transformer-winding which is energized byconduction in the secondary winding 108 of the power transformer 110.The function of the power limit circuit 188 is to supply a controlsignal, via gate 187, to the blocking oscillator to inhibit itsoperation whenever the output diode 126 is conducting. The power limitcircuit in severe overload conditions of the converter overrides thecurrent regulation control of the converter as described hereinbelow.

CURRENT REGULATION The output current of the converter is monitored bythe current signal amplifier 103 whose input terminals are connectedacross the current sensing resistor 129. The output of the currentsignal amplifier 103 is a voltage signal which is directly proportionalto the converter output current. The current signal amplifier 103applies this voltage signal to the current regulator circuit 104. Whenthe current through the sensing resistor 129 exceeds the convertersrated current, the signal applied by the current signal amplifier 103 tothe current regulator circuit 104 causes it to respond and generate anoutput signal. The output signal of the current regulator circuit 104 isapplied to the gate where it functions to override the output of thevoltage regulating amplifier 111. This regulation signal is applied, vialead 186, to the comparator circuit 113. The comparator circuit 113responds and supplies signals to current regulation characteristic isshown by the waveform b in FIG. 2.

HIGH VOLTAGE SHUTDOWN A high voltage monitor circuit 114 has its inputconnected to the output terminals 131 and 132, via leads 191 and 192.The high voltage monitor circuit 1 14 continuously monitors theconverter output voltage. The occurrence of a high output voltage abovea predetermined threshold causes the high voltage monitorning circuit114 to generate an output signal. This output signal is applied via lead193 to operate the shutdown circuit 194. The shutdown circuit 194includes an output connected to a disconnect circuit 150. The disconnectcircuit 150 responds to the output of the shutdown circuit 194 andapplies a signal, via lead 151, to activate a circuit-breaking device154 to disconnect the converter from the load or bus. Another outputlead 155 utilizes the output of the shutdown circuit 194 to shut downthe blocking oscillator 135.

The high voltage threshold at which the high voltage monitoring circuit114 operates is a function of the output load current of the converter.This may be readily ascertained by inspection of the voltagecharacteristic of the voltage curve c as shown in FIG. 2.

When two or more converters are operated in parallel, a failure of thevoltage regulation control in any one converter applies an overvoltageto the output terminals of all the converters connected in parallel.Normally the malfunctioning converter supplies a higher output current.Selective shutdown of the individual converter causing the high voltagecondition is required. This selection is made by generating a biassignal proportional to the converter current and using it to control thethreshold at which the high voltage monitor circuit 114 responds. Thisbias signal is supplied to the high voltage monitoring circuit, via lead153, from the current signal amplifier 101. This bias signal supplied bythe current signal amplifier 101 produces in the high voltage monitorcircuit 114 a linearly decreasing high voltage shutdown thresholdproportional to the increasing converter output current. Hence, themalfunctioning converter has a lower shutdown threshold since its outputcurrentis larger than the other converters. It is apparent from theforegoing that the current supplied by the other normally functioningconverters connected to the same bus is reduced and their high voltageshutdown threshold is consequently increased as their output current isdecreased. Therefore, the individual converter responsive for theovervoltage condition has the lowest voltage shutdown threshold andhence is selectively shut down.

The output signal of the high voltage monitor circuit 114 is applieddirectly, via lead 193, to the shutdown circuit 194. Its output signalactivates the shutdown circuit 194 which operates to inhibit theblocking oscillator 135 and activates the disconnect circuit 150. Thisshuts down the converter circuit and isolates it from the load.

CURRENT CONTRIBUTION CONTROL The otuput of the current signal amplifier101 is also connected, via leads 153 and'195, to a light load voltageuptilt circuit 102. The voltage'uptilt characteristic is shown bywaveform d in FIG. 2. As is apparent from inspection of the voltagewaveform d in FIG. 2, the light load voltage uptilt circuit 102 operatesto provide an increased slope to the voltage regulation characteristicof the con erter by modifying the response of the voltage regulatingamplifier 111.

Normally a plurality of converters connected in parallel to a common buswill not share equally in supplying the bus current. Some converters maydeliver no current at all even though they are operating at theirregulated voltage. According to the invention, as the current output ofthe converter decreases below some specified threshold, the regulatedvoltage characteristic of the converter is increased linearly. This isto insure that the converter will supply at least a minimum current tothe common bus.

The light load voltage uptilt circuit 102 is responsive to the output ofthe current signal amplifier 101. As the input signal supplied by thecurrentsignal amplifier 101 decreases below some specified threshold,the light load voltage uptilt circuit 102 generates a bias signal whichis applied to the voltage regulating amplifier 111. This bias signal isinversely proportional to the output current of the converter. This biassignal produces a linar increase in the voltage regulation threshold ofthe converter. Hence, the voltage regulating amplifier 111 now regulatesthe output of the converter at a higher voltage. The higher voltageregulation threshold insures that the converter will supply at least aminimum output current to the bus. The output of the light load voltageuptilt circuit 102 may be utilized to operate an alarm since a converterat a very low current output may be operating in an abnormal conditionor may have failed.

OVERCURRENT SHUTDOWN The output current of the converter through thecurrent sensing resistor 129 is sensed by the current signal amplifier101 to operate the overcurrent protection shutdown of the converter 132.The current signal amplifier 101 produces an output signal directlyproportional to the converters output current. The output of the currentsignal amplifier 103 is applied to the overcurrent monitor circuit 106.The overcurrent monitor circuit 106 includes a time delay to preventnuisance shutdowns caused by monentary overcurrent current outputs, andalso to allow the overcurrent a sufficient duration to operate anyprotective device on the bus. Should the converter output current exceedthe shutdown threshold for a sufficient period of time, the output ofthe current signal amplifier 101 is sufficient to operate theovercurrent monitor circuit 106.

The otuput of the overcurrent monitor circuit 106 is directly applied,via lead 189, to operate the shutdown circuit 194. The shutdown circuitoutput activates the disconnect circuit which in turn operates thecircuit-breaking device 154 to disconnect the converter from the bus orload attached to the output terminal 131 and 132.

As described hereinabove, the output of the converter is currentregulated and hence the overcurrent condition does not normally occur.Even though the current output is regulated, it is desirable to shutdown the circuit should a sustained short circuit occur on the bus. Theovercurrent protectionis also desirable as a backup system should thenormal current regulation circuitry fail to operate.

In such overcurrent conditions, the current conducted through the outputdiode 126, although limited, has a long duration. The power limitcircuit 188 inhibits the operation of the blocking oscillator as long asthe output diode 126 conducts. During severe overload conditions,therefore, the power limit circuit 188 inhibits the operation of theblocking oscillator 135 in response to the sustained conduction throughthe output diode 126.

The output voltage of the bus also decreases in response to a shortcircuit or fault. This decrease is monitored by the comparator 113 whichincludes a reference voltage device which renders it inactive at lowconverter output voltages. When this low voltage occurs, the normalcurrent regulation of the converter is rendered inactive, and theconverter circuit operates in response to the overcurrent monitorcircuit 106. Hence, a shutdown signal is supplied from the overcurrentmonitor 106, via lead 189, to the shutdown circuit 194 to shut down theconverter by inhibiting the blocking oscillator 135. The shutdowncircuit 194 also activates the disconnect circuit 150 which in turnactivates the circuit-breaking device 154 to disconnect the converterfrom the bus or the load. The overcurrent shutdown characteristic of theconverter may be readily ascertained by reference to waveform e in FIG.2.

REVERSE CURRENT PROTECTION I If a short circuit or fault internal to theconverter should occur, the bus will supply currentin a reversedirection to the converter. The current signal amplifier 103 is utilizedto monitor the current direction through the sensing resistor 129.Should the current signal amplifier 103 detect a reverse current, asignal is provided to the reverse current level detector 105 via thegate 167. Upon the detection of a reverse current signal, the reversecurrent level detector 105 applies a signal, via lead 198, to theshutdown circuit 194. The shutdown circuit 194 operates to inhibit theblocking oscillator and shut down the converter. The output of theshutdown circuit 194 also activates the disconnect circuit 150 whichoperates the circuit-breaking device 154 to disconnect the converterfrom the bus or the load.

The input to the reverse current monitor circuit 107 is shunted acrossthe resistor 133 which is connected in series with thefilter capacitor130. Should the current therein increase above a specified thresholdindicating a failure of the filter capacitor 130, the reverse currentmonitor 107 generates an output signal. The output signal of the reversecurrent monitor circuit 107 is applied, via gate 167, to operate thereverse current level detector 105. The outputof the reverse currentlevel detector 105 is applied, via lead198, to operate the shutdowncircuit 194 and disconnect the converter from the bus or the load asdescribed hereinabove.

The circuit schematic illustrated in FIG. 3 shows an embodiment of theDC to DC converter, according to the invention, utilizing operationalamplifiers. The power circuit portion of the converter operatessubstantially the same as was described with reference to the convertershown in FIG. 1. A DC voltage source is coupled to the input terminals221 and 222 and is connected to the switching devices of the inverter,via an input filter comprising the inductor 223 and the capacitor 217.Thefiltered voltage ofthe DC voltage source is applied to the two SCRdiodes 215 and 216 each of which is connected in a resonant LC networkas described hereinabove. The SCR diodes 215 and 216 are driven bysimultaneous pulse signals applied to their trigger input terminals bythe blocking oscillator 235.

The blocking oscillator 235 is an astable free running oscillator. Thefrequency of switching of the SCR diodes 215 and 216 is controlled byperiodically inhibiting the operation of the blocking oscillator 235.The input to inhibit the blocking oscillator 235 is controlled by theconduction state of the transistor 236. The blocking oscillator 235 canbe frequency controlled by having the transistor 236 periodicallyconduct or the converter may be shut down by having the transistor 236permanently conduct.

The current flow through the diodes 215 and 216 stores energy in thecore of the transformer 210. This energy is periodically discharged, viathe secondary winding 208 through the output diode 226 to the outputterminals 231 and 232. A tertiary winding 220 is wound on thetransformer core of the transformer 210. The tertiary winding 220 isconnected, via a diode 237, and lead 238 to the transistor 236 andfunctions as a power limit control. This power limit control is utilizedto inhibit the operation of the blocking oscillator 235 whenever theoutput diode 226 is conducting. This power limit control performs aprotection function by inhibiting the operation of the blockingoscillator 235 as long as output current is flowing through diode 226.Hence, the converter may be operated into a shorted output withoutdamage to its circuit components.

Operational amplifiers are utilized as feedback regu latory controlcircuits to regulate the output of the converter and provide protectionagainst overcurrent and overvoltage conditions. These operationalamplifiers monitor the various output signals of the converter andrespond to control the frequency of the blocking oscillator or inhibitits operation on disconnect the converter from the load, depending uponthe response needed to secure the desired reguation or protection.

VOLTAGE REGULATION The output voltage of the converter is regulated atsome predetermined value. This voltage is monitored by a voltageregulation operational amplifier 211. One input of the operationalamplifier 211 is connected to the output terminal 231 of the convertervia lead 242.

A reference voltage source 28 is also connected to this same input. Theoutput terminal 231 of the converter is connected to a reference ground35. The other input terminal of the operational amplifier 211 isconnected to the reference ground 35a which is the same potential asreference ground 35.

The sum of this reference voltage and the output voltage is applied tothe one input of the voltage regulation operational amplifier 211. Thevoltage regulation operational amplifier 211 amplifies this signal andapplies it to a comparator operational amplifier 213, via the diode 243and resistor 246.

The terminal 207 of the secondary winding 208 is coupled, via the leads244 and 245, to a synchronizing operational amplifier 212. Since thisconnection precedes the output filter of the converter, thesynchronizing operational amplifier 212 amplifies the output ripplesignal of the converter. The amplified ripple output of thesynchronizing operational amplifier 212 is applied to the other input ofthe comparator operational amplifier 213, via resistor 247.Thecomparator operational amplifier 213 responds-in a binary fashion to theoutputs of the synchronizing operational amplifier 212 and the voltageregulation operational amplifier 21 1. If the output of the voltageregulation operational-amplifier'2l1 exceeds the output of thesynchronizing operational amplifier 212, the comparator operationalamplifier 213 applies a signal, via diode 248 and resistor 249, to thebase of transistor 236. The transistor 236 is biased conducting inresponse to this signal and its output to the blocking oscillator 235inhibits its operation. Hence, it is apparent that if the output voltageexceeds a certain regulated value, the voltage regulation circuitrygenerates a signal to inhibit the operation of the blocking oscillator235 driving the switching devices. The operation of the blockingoscillator is inhibited until the output voltage of the converter isreduced to its regulated value.

CURRENT REGULATION The output current of the converter flows through asensing resistor 229. The sensing resistor 229 has a very low impedanceso as not to impair the efficiency of the converter. The low levelvoltage across the sensing resistor 229 is monitored by the currentsignal operational amplifier 203 whose input terminals shunt the sensingresistor 229. The low level voltage of the resistor 229 is amplified bythe current regulating operational amplifier 204. A reference voltagesource 21 is also connected to the input of the current regulatingoperational amplifier 204 to establish a response threshold. The currentsignal operational amplifier 203 inverts the voltage detected across thesensing resistor 229. The voltage supplied by the reference voltagesource 21 is of opposite polarity to the output of the current signaloperational amplifier 203. The current regulating operational amplifier.204 responds to the weighted sum of these two voltages. The outputvoltage signal of the current regulating operational amplifier 203 isapplied, via a diode 251 and the resistor 246 to the input of thecomparator operational amplifier 213. The weighting is such that whenthe full current load of the converter circuit occurs the voltageregulation amplifier 211 no longer controls the output state of thecomparator operational amplifier 231 and the converter operates in acurrent regulation mode. The output of the comparator operationalamplifier 213 is applied, via a diode 248 and resistor 249, to controlthe conductivity of the transistor 236 and in turn control the blockingoscillator 235, asdescribed hereinabove with reference to the voltageregulation circuitry.

A feature of the current regulation circuitry important to thehereinbelow description of the overcurrent protection is that thecurrent regulation does not work at output voltages below a minimumthreshold. As long as the output voltage is above a certain threshold,as shown in FIG. 2 by waveform g, the current regulation circuitry isfunctional. When it drops below this level and the output currentincreases to a full load current, the voltage breakdown diode 299connected to the input of th comparator operational amplifier 213 breaksdown and establishes a minimumvoltage threshold below which the currentregulation ceases to operate. At this voltage threshold the power limitcontrol described hereinabove takes over to control the blockingoscillator and continues to function until the resulting overcurrenteither clears a protective device on the current bus or activates theovercurrent shutdown circuit described hereinbeow.

HIGH VOLTAGE SHUTDOWN The output voltage of the converter is monitoredby a high voltage monitoring operational amplifier 214. The input lead252 of the high voltage monitoring operational amplifier 214 isconnected to output terminal 232 of the converter and to a referencevoltage source 22. The output voltage and the reference voltage aresummed. The high voltage monitoring operational amplifier 214 is a veryhigh gain amplifier and when this summed voltage reaches a certainthreshold it operates in a binary fashion. When this occurs the highvoltage monitoring operational amplifier 214 applies a signal, via diode254 to break down the breakdown diode 255. This signal triggers the SCRdiode 257 into its conducting state. The sCR diode 257 is connected tothe blocking oscillator 235 and inhibits the operation of the blockingoscillator 235. The anode of the SCR diode 257 is also connected, vialead 271, to a disconnect circuit 270 which is activated when the sCRdiode 257 conducts. The output of the disconnect circuit 270 is applied,via lead 272, to activate the circuit breaking device 273 whichdisconnects the converter from the load. The separate arrangementdisclosed here to inhibit the operation of the blocking oscillator 235is in cluded to provide added protection against possible failure of thenormal inhibit control applied via the transistor 236. The SCR diode 257when conducting diverts the charging current of the timing capacitor inthe oscillatory circuitry of the blocking oscillator 235.

As described hereinabove, when converters are connected in parallel anyhigh voltage conditon which occurs will be common to all the converters.Hence, it is necessary to determine which converter is malfunctioning.This is determined by finding the converter applying an overload currentto the common bus. A current signal operational amplifier 201 monitorsthe output current flowing through the sensing resistor 229. The outputvoltage of the current signal operational amplifier 201 is proportionalto the converter output current. This output is applied to the input ofthe high voltage monitoring operational amplifier 214, via lead 253 toserve as a bias signal. This bias signal modifies the voltage level atwhich the high gain high voltage monitoring operational amplifier 214responds. This bias signal produces the linearly decreasing high voltageshutdown threshold that is shown by waveform c in FlG. 2. As is apparentfrom this diagram, the shutdown threshold of the converter isconsiderably less at full rated current than at a smaller current.

LOW CURRENT VOLTAGE UPTILT As described hereinabove, while the voltageregulation control constrains each of the converters connected inparallel to operate at the same voltage, they will not necessarilycontribute the same current to the common bus or load. It is desirable,therefore, to contrain each converter to contribute at least a minimumcurrent ot the common bus or load. Forcing a minimum currentcontribution from each converter provides an easy and inexpensive meansof detecting a failed or poorly adjusted converter. To achieve thisresult the output current of the converter flowing through the sensingresistor 229 is monitored by the current signal operational amplifier201. The amplified voltage output of the current signal opertionalamplifier 202 is applied to a voltage regulating low current signaloperational amplifier 202. A reference voltage source 26 is connected tothe other input of the low current signal amplifier 202 to establish athreshold'at which it operates. If the voltage output of the currentsignal operational amplifier 201 decreases below the threshold voltageestablished by the reference voltage source 26, the operationalamplifier 202 becomes active and applies an output bias signal linearlyrelated to the current flowing through the sensing resistor 229 to theinput of the voltage regulating operational amplifier 211. This biassignal is inversely proportional to the output current and modifies theoperating response of the voltage regulating operational amplifier 211so that the voltage at which the regulation circuitry regulates isincreased. The output of the voltage regulation operational amplifier211 is utilized in the same manner as described hereinabove to regualtethe converters output voltage. By increasing the regulated voltage atlow current levels, as shown by waveform d in FIG. 2, each converter isconstrained to supply at least a minimum current to the common bus oroutput load.

OVERCURRENT SHUTDOWN As described hereinabove, the current regulationportion of the converter does not function below a certain outputvoltage level of the converter. When the converter output voltage dropsbelow this level, the output current of the converter is freeto increasebeyond the regulated value. At this point the overcurrent shutdownprotection may operate to shut down the converter if the output currentexceeds a certain threshold at which overcurrent protection is desired.The output current of-the converter is monitored by the current signaloperational amplifier 201 as described above and its output voltagesignal proportional to this current is applied to an overcurrentmonitoring operational amplifier 206. A reference voltage source 24 isconnected to the other input of the operational amplifier 206 toestablish the input threshold at which the overcurrent monitoringoperational amplifier 206 responds. The operational amplifier 206includes a feedback capacitor 264 to introduce a delay into itsresponse. This delay prevents the overcurrent protection from respondingto momentary overloads. If the overcurrent persists beyond the delayestablished by the feedback capacitor 264, the overcurrent monitoringoperational amplifier 206 applies-an output signal, via diode 265 andthe breakdown diode 255, to the trigger input of the SCR diode 257. Thisbiases the SCR diode 257 into-a conducting state and shuts down theblocking oscillator 23S, and'operates the disconnect circuit 270 todisconnect the converter from the load.

REVERSE CURRENT SHUTDOWN Should an internal short circuit or fault occurwithin the converter, current will flow from the load and from the otherparallel connected converters into the malfunctioning converter. Reversecurrent protection is provided by monitoring the direction of current inthe output circuit of the converter. The output polarity of the currentsignal operational amplifier 203 is indicative of the direction ofcurrent through the sensing resistor 229. The current flowing into theoutput filter capacitor 230 is monitored by sensing the voltage acrossthe series connected resistor 233. The voltage across this resistor isapplied to a reverse current monitoring operational amplifier 277. Theoutputs of the current signal operational amplifier 203 and the reversecurrent monitoring operational amplifier 277 are coml 2 bined in a diodebridge 267 and applied to a reverse current level detecting operationalamplifier 205.

The diode bridge 267 is poled to transmit only voltage signalsindicative of reverse currents in the resistors 229 and 233.

The operational amplifier 205 includes a feedback capacitor 276 tointroduce a time delay so it does not respond to momentary reversecurrents. A reference voltage source 29 is connected to the operationalamplifier 205 to set the threshold at which the delay operates. When thereverse current detected is sufficient in magnitude and duration tooperate the reverse current level detecting operational amplifier 205,it applies an output signal via the diode 268 and the breakdown diode255 to operate the SCR diode 257 and hence shut down the blockingoscillator 235 and disconnect the converter load by actuating thedisconnect circuit 270.

It is apparent from the foregoing description that many various schemesand approaches may be improvised to apply the features of applicantsinvention to permit converters to operate in parallel. While the abovedescription has been confined to one embodiment implementing thesefeatures, many alternative ways will suggest themselves to those skilledin the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a converter circuit, means to control the output of said convertercomprising, means to monitor the voltage output of said converter, meansto monitorthe output current of said converter, voltage comparison meansconnected to said means'to monitor the voltage output, means toestablish a voltage threshold connected to said voltage comparisonmeans, said means to establish a voltage threshold being connected toand responsive to said means to monitor the output current, and meansresponsive to said voltage comparison menas to disable said converterwhereby the voltage threshold at which said converter is disabled isproportional to the output current of the converter.

2. In a converter circuit as defined in claim 1 further including, meansto regulate the voltage output of said converter including a regulationreference voltage source, means responsive to said means to monitor theoutput current to modify the voltage of the regulation reference voltagesource, said means to modify the voltage of the regulationreferencevoltage source including threshold means to establish a currentthreshold at which it becomes operative.

3. In a power supply circuit 'a high voltage protective shutdown circuitcomprising, means to monitor the voltage output of said power supply, asource of reference voltage, means to compare the voltage sensed by saidmeans to monitor the voltage output with said reference voltage, whereinthe'improvement comprises, means to monitor the current output of saidpower supply, means to generate a bias signal proportional to thecurrent sensed by the means to monitor the current output, means toapply said bias signal to said means to compare to alter its thresholdof operation, means to utilize the output of said means to compare toshut down said power supply whereby the voltage threshold at which saidshutdown circuit operates is altered in proportion to the output currentof said power supply.

4. [n a power supply as defined in claim 3 further including, means toregulate the voltage output of said power supply, wherein theimprovement further comprises, means responsive to said means to monitorthe current output to detect low' current outputs, and means to increasethe voltage level at which said means to regulate regulates the outputvoltage upon the detection of a low current output.

5. In a DC to DC converter including a switching device, oscillatorymeans to drive said switching device, and output means to connect to aload to be energized wherein the improvement comprises, a high voltageprotection circuit comprising a first operational amplifier connected ina high gain summing mode and having an input to establish its voltagethreshold of operation, the output means of said converter being coupledto the input of said first operational amplifier, a reference voltagesource connected to the input of said first operational amplifier toestablish its voltage threshold, current sensing means in said outputmeans, a second operational amplifier connected in an amplifying modeand having its input coupled to said current sensing means, the otuputof said second operational amplifier being connected to the input ofsaid first operational amplifier to establish the voltage threshold ofsaid first operational amplifier, and means to inhibit the operation ofsaid oscillatory means including means to dis- 14 connect said converterfrom its load, the output of said first operational amplifier beingconnected to said means to inhibit, whereby the voltage threshold atwhich said first operational amplifier is responsive is proportional tothe magnitude of the output current of the converter.

6. A DC to DC converter as defined in claim 5 further including, a thirdoperational amplifier connected in summing mode and having its summinginput connected to said output means, means to utilize the output ofsaid third operational amplifier to regulate the voltage of the outputmeans of said converter, wherein the improvement comprises, a fourthoperational amplifier connected in an amplifying mode, a second'reference voltage source connected to an input of said fourthoperational amplifier to establish a threshold response, the output ofsaid operational amplifier being connected to an input of said fourthoperational amplifier, the output of said fourth operational amplifierbeing coupled to an input of said third operational amplifier whereinthe regulated voltage value is modified in response to the magnitude ofthe current in said output means.

1. In a converter circuit, means to control the output of said convertercomprising, means to monitor the voltage output of said converter, meansto monitor the output current of said converter, voltage comparisonmeans connected to said means to monitor the voltage output, means toestablish a voltage threshold connected to said voltage comparisonmeans, said means to establish a voltage threshold being connected toand responsive to said means to monitor the output current, and meansresponsive to said voltage comparison menas to disable said converterwhereby the voltage threshold at which said converter is disabled isproportional to the output current of the converter.
 2. In a convertercircuit as defined in claim 1 further including, means to regulate thevoltage output of said converter including a regulation referencevoltage source, means responsive to said means to monitor the outputcurrent to modify the voltage of the regulation reference voltagesource, said means to modify the voltage of the regulation referencevoltage source including threshold means to establish a currentthreshold at which it becomes operative.
 3. In a power supply circuit ahigh voltage protective shutdown circuit comprising, means to monitorthe voltage output of said power supply, a source of reference voltage,means to compare the voltage sensed by said means to monitor the voltageoutput with said reference voltage, wherein the improvement comprises,means to monitor the current output of said power supply, means togenerate a bias signal proportional to the current sensed by the meansto monitor the current output, means to apply said bias signal to saidmeans to compare to alter its threshold of operation, means to utilizethe output of said means to compare to shut down said power supplywhereby the voltage threshold at which said shutdown circuit operates isaltered in proportion to the output current of said power supply.
 4. Ina power supply as defined in claim 3 further including, means toregulate the voltage output of said power supply, wherein theimprovement further comprises, means responsive to said means to monitorthe current output to detect low current outputs, and means to increasethe voltage level at which said means to regulate regulates the outputvoltage upon the detection of a low current output.
 5. In a DC to DCconverter including a switching device, oscillatory means to drive saidswitching device, and output means to connect to a load to be energizedwherein the improvement comprises, a high voltage protection circuitcomprising a first operational amplifier connected in a high gainsumming mode and having an input to establish its voltage threshold ofoperation, the output means of said converter being coupled to the inputof said first operational amplifier, a reference voltage sourceconnected to the input of said first operational amplifier to establishits voltage threshold, current sensing means in said output means, asecond operational amplifier connected in an amplifying mode and havingits input coupled to said current sensing means, the otuput of saidsecond operational amplifier being connected to the input of said firstoperational amplifier to establish the voltage threshold of said firstoperational amplifier, and means to inhibit the operation of saidoscillatory means including means to disconnect said converter from itsload, the output of said first operational amplifier being connected tosaid means to inhibit, whereby the voltage threshold at which said firstoperational amplifier is responsive is proportional to the magnitude ofthe output current of the converter.
 6. A DC to DC converter as definedin claim 5 further including, a third operational amplifier connected insumming mode and having its summing input connected to said outputmeans, means to utilize the output of said third operational amplifierto reguLate the voltage of the output means of said converter, whereinthe improvement comprises, a fourth operational amplifier connected inan amplifying mode, a second reference voltage source connected to aninput of said fourth operational amplifier to establish a thresholdresponse, the output of said operational amplifier being connected to aninput of said fourth operational amplifier, the output of said fourthoperational amplifier being coupled to an input of said thirdoperational amplifier wherein the regulated voltage value is modified inresponse to the magnitude of the current in said output means.